German Patent No. 42 32 950 describes a device for controlling a position of an operating element using an electric motor drive unit controlled by a signal processing arrangement. An incremental position sensor for determining the actual position of the operating element is arranged in the drive unit. The incremental position sensor comprises a magnet wheel, arranged on a motor armature shaft, with a plurality of segments with differing magnetizations, as well as magnetic field sensors that are stationary in relation to the frame and are used as signal receivers that convert the north and south magnetization prevailing at the point of the magnetic field sensor into signals with two different levels. The magnet wheel moves by the amount of one segment with respect to the magnetic field sensor between two level changes of one of the magnetic field sensors. The actual position of the operating element with respect to a calibrating position is determined by counting the different successive levels and incrementing or decrementing the counter depending on the direction of rotation of the motor armature shaft.
If the power supply of the magnetic field sensors is turned off (for example, in order to save energy when the motor armature shaft is at rest), erroneous counts may occur which cause the incremental determination of the actual position of the actuator part to be erroneous also. As a result, a position determination recalibration will most likely have to be performed.